320 



MANUAL OF BOTANY 



the wood of vascular bundles. In size they vary from the ^ Jij to 

 hAtt of 8,n inch in diameter. Tlie average size is about the xcfcc- 



c. Annular Vessels. — In these vessels the thickening matter is 

 arranged in the form of rings more or less regularly disposed upon 

 their inner surface (figs. 688, r; 689, and 690). Sometimes the 

 whole of the vessel presents this ringed appearance [figs. 689 and 

 690), while in other vessels we find two rings connected by one 

 or more turns of a spiral, the two forms irregularly alternating 

 with each other (fig. 691). In size they vary from about jj^ to 

 gjjy of an inch in diameter. Annular vessels occur especially 

 in the wood of the fibro-vascular bundles of the stems of soft, 

 rapidly growing herbaceous plants. 



cl. Reticulated Vessels. — In these vessels the convolutions 



Fiu. 692. 



Fig. 693. 



Fig. 694. 



Fig. 692. Eeticulated vessel. IHg. 693. Prismatic scaliu-ifoiin vessels of 



j^ Fern. Fig. 694. Cylindrical scalariform vessels of the Yine. 



are more or less irregular, and connected in various ways by 

 cross or oblique bands, so as to produce a. branched or netted 

 appearance (fig. 692). 



L. Scalariforna Vessels. — The peculiar appearance of tliese 

 vessels is owing to their walls being marked by elongated trans- 

 verse pits or lines, arranged over one another like the steps of a 

 ladder, -^hence their name (fixjs. 693 and 694). They are some- 

 times cylindrical tubes like the other vessels, as in the Vine 

 (fig. 694), and in many other Dicotyledons, in which condition 

 they resemble modifications o£ reticulated vessels ; but in their 

 more perfect state, scalariform vessels assume a prismatic form, 

 as in Ferns (fig. 693). 



The scalariform markings are often caused in the same way 

 as those of bordered pits, the thin places, instead of being small 



